Production of silicon nitride sintered body

ABSTRACT

A process for manufacturing a silicon nitride sintered body comprises molding a mixed powder of a silicon nitride starting material and a sintering aid and firing a thus obtained molding. The firing is carried out in an N 2  atmosphere or a mixed atmosphere of N 2  and an inert gas to which CO 2  or a mixed gas of CO 2  and CO is added. Thereby, an O 2  partial pressure is increased to restrain the evaporation of SiO 2  and nitriding of SiO 2  during the firing. The silicon nitride sintered body suffers almost no deterioration of the fired surface resulting from the evaporation of SiO 2  and the nitriding of SiO 2 , and exhibits substantially equal four point bending strength and oxidation resistance with respect to the fired surface and the inside thereof.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a process suitable for manufacturing a silicon nitride sintered body.

(2) Related Art Statement

According to a conventional process for producing a silicon nitride sintered body, a sintering aid which forms a liquid phase in grain boundaries, such as Y₂ O₃, Al₂ O₃, MgO, etc., is added to a raw material powder of silicon nitride, and a molding obtained from a resulting mixture is fired in an N₂ atmosphere or a mixed atmosphere of N₂ and an inert gas or in such an atmosphere under pressure. For instance, Japanese Patent Publication No. 58-49,509 discloses a process for performing firing in an N₂ atmosphere under pressure or in a mixed atmosphere of N₂ and an inert gas under pressure.

In these cases, a carbonaceous heater or a firing jig is ordinarily used, and the atmosphere is an N₂ atmosphere or a mixed atmosphere with a low O₂ partial pressure in which O₂ contained as an impurity in N₂ gas is reduced.

By the way, SiO₂ which is inherently contained in an oxide additive and a silicon nitride raw material and serves as a sintering aid forms glass in grain boundaries through reaction and effectively performs densification of a structure and formation of a fine structure. However, in the above-mentioned conventional processes, when the silicon nitride molding is fired in N₂ atmosphere or N2pressurizing atmosphere with a low O₂ partial pressure, as shown in the following expressions (1) and (2), the oxide additive and SiO₂ evaporate from the glass phase, or are nitrided. Thereby, the ratio between O and N in the glass phase varies so that the composition of the glass phase at the grain boundary changes.

    Evaporation reaction of SiO.sub.2 :SiO.sub.2 ⃡SiO+1/2O.sub.2 ( 1)

    Nitriding reaction of SiO.sub.2 :3SiO.sub.2 +2N.sub.2 ⃡Si.sub.3 N.sub.4 +3O.sub.2                                         ( 2)

For this reason, the conventional process has the drawback that a sufficiently densified silicon nitride cannot be obtained or difference occurs in fine structure between the surface at which evaporation easily takes place and the inside portion at which the evaporation is difficult, thereby deteriorating the properties of the fired face.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to eliminate the above-mentioned problems and to provide a process for producing a silicon nitride sintered body which suffers almost no deterioration of a fired surface resulting from the evaporation and nitriding reaction of SiO₂.

According to the present invention, there is a provision of a process for producing a silicon nitride sintered body, which comprises molding a mixed powder of a silicon nitride starting material powder and a sintering aid, and firing a resulting molding in an N₂ atmosphere or a mixed atmosphere of N₂ and an inert gas in which CO₂ or a mixed gas of CO₂ and CO is added so that an O₂ partial pressure is increased to restrain the evaporation of SiO₂ and the nitriding of SiO₂ when the molding is fired.

By so doing, SiO₂ is prevented from evaporating and being nitrided by increasing the partial pressure of O₂ in the N₂ atmosphere or the mixed atmosphere of N₂ and the inert gas.

These and other objects, features and advantages of the present invention will be well understood from the following description of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A preferable O₂ partial pressure can be selectively determined depending upon an equilibrium O₂ partial pressure, an evaporation speed, and a nitriding speed in the evaporation reaction of SiO₂ and the nitriding reaction of SiO₂, and a firing time at a firing temperature.

As a way of increasing this O₂ partial pressure, CO₂ or a mixed gas of CO₂ and CO is further mixed into the N₂ atmosphere or the mixed atmosphere of N₂ and the inert gas. Thereby, the O₂ partial pressure is increased by utilizing the equilibrium O₂ partial pressure resulting from the CO₂ dissociation reaction shown by the following expressions (3) and (4).

Equilibrium reactions of CO₂ :

    CO.sub.2 ⃡CO+1/2O.sub.2                        (3)

    CO.sub.2 ⃡C+O.sub.2                            (4)

The reason why CO₂ or the mixed gas of CO₂ or CO is selected as an addition gas to enhance the O₂ partial pressure in the present invention is that by controlling the O₂ partial pressure through the CO₂ dissociation reactions, damages of a carbonaceous heater, a jig, etc. frequently used for firing the silicon nitride sinterable body are reduced and that a range of the O₂ partial pressure is appropriately selected and easy to be controlled.

The reason why 0.001% or more of CO₂ is favorably mixed into the N₂ atmosphere or the mixed atmosphere of N₂ and the inert gas is that no effect is recognized in the case of less than 0.001% because less than 0.001% becomes lower than the content of impurities usually contained in N₂ gas.

On the other hand, if the mixing amount of CO₂ is too much, the O₂ partial pressure becomes too high so that the oxidation reaction of Si₃ N₄ unfavorably becomes conspicuous. The reason why the total pressure of the N₂ atmosphere or the mixed atmosphere is favorably set at not less than 1 atm. is because the oxidation reaction of Si₃ N₄ can be restrained and firing can be done by increasing the N₂ partial pressure even when the O₂ partial pressure is high. Thus, the evaporation of the SiO₂ can be effectively restrained.

In summarizing the above, the O₂ partial pressure is raised to restrain the evaporation of SiO₂. The O₂ partial pressure is increased and balanced with the N₂ partial pressure to restrain the nitriding of SiO₂. Further, the N₂ partial pressure is increased and balanced with the O₂ partial pressure to restrain the oxidation of Si₃ N₄.

Further, when CO is mixed into the firing atmosphere together with CO₂, a CO formation reaction shown in the following formula (5) is made difficult to take place, and a consumed amount of carbon (C) is decreased to reduce damages of the carbonaceous heater, jig, etc. frequently used in the firing of the silicon nitride sinterable body.

CO formation reaction:

    CO.sub.2 +C→2CO                                     (5)

It is preferable that a mixing rate of CO is greater than that of CO₂. However, if CO is in a pressure higher than an equilibrium partial pressure of CO in the reaction shown by the formula (5), a formation reaction of CO₂ and C reverse to the reaction shown in the formula (5) takes place so that a produced C deposits on the silicon nitride sintered body or reacts with SiO₂ in the silicon nitride sinterable body. Thus, the pressure higher than the CO equilibrium partial pressure is unfavorable.

In addition, in order to reduce damages of the carbonaceous heater, jig, etc. in the firing furnace for the silicon nitride sinterable body and control the O₂ partial pressure in the firing furnace to an appropriate range, it may be that an N₂ gas or a mixed gas of N₂ and an inert gas which contains CO and CO₂ is prepared by passing N₂ gas or a mixed gas of N₂ and the inert gas which is mixed with gases of O₂, H₂ O, air, CO₂, etc. through a heater placing a consumable carbon source therein, and is then introduced into the firing furnace for the silicon nitride sinterable body. As a matter of course, it may be that a gas containing CO and CO₂ is prepared by passing a gas comprising O₂, H₂ O, air, CO₂, etc. through the heater placing the carbon source therein, and mixed into an N₂ gas or a mixed gas of N₂ and an inert gas, which is then introduced into the silicon nitride sinterable body-firing furnace.

Moreover, in order to reduce damages of the carbonaceous heater, jig, etc. in the firing furnace for the silicon nitride sinterable body and control the O₂ partial pressure in the firing furnace to the appropriate range, it may be that an N₂ gas or a mixed gas of N₂ and an inert gas containing CO and CO₂ is prepared by reacting an N₂ gas or a mixed gas of N₂ and the inert gas which contains gases of O₂, H₂ O, air, CO₂, etc. with a consumable carbon source at an initial stage location of a gas introducing path of the firing furnace, and then introduced into a certain location of the heater of the jig in the firing furnace.

The process for manufacturing the silicon nitride sintered body according to the present invention will be explained in more detail.

First, the silicon nitride powdery starting material is prepared. The silicon nitride powdery starting material is composed of a formulated powder of a silicon nitride raw material powder and a sintering aid. Y₂ O₃, MgO, Al₂ O₃, etc. is added as a sintering aid as it is or in a form of an aqueous solution.

Next, the above silicon nitride starting material powder is crushed and mixed by means of a mill using media. The mills of a wet type and a dry type may be both used. For instance, a ball mill, an attrition mill, a vibration mill, etc. may be used. Then, a thus obtained molding powder is molded by a dry type press, an injection molding, a slip casting, etc., thereby, obtaining a desired molding.

The thus obtained molding is fired in an N₂ atmosphere or a mixed atmosphere of N₂ and an inert gas into which CO₂ or a mixed gas of CO₂ and CO is added. The firing temperature is preferably in a temperature range from 1,600° C. to 2,000° C. The addition amount of CO₂ to N₂ is preferably in a range not less than 0.001%. It is preferable that the total pressure of the N₂ atmosphere or the mixed atmosphere is not less than 1 atm. The desired silicon nitride sintered body can be obtained by the above-mentioned process.

The present invention will be explained below in more detail based on the following examples, which are merely given in illustration thereof, but should never be interpreted to limit the scope of the invention.

EXAMPLE 1

A preparation powder was formulated by adding a sintering aid to a silicon nitride powdery starting material of 97.1% by weight in purity with a content of oxygen being 1.5% by weight at a rate shown in Table 1. After the preparation powder was mixed and crushed by means of a water wet type ball mill, the powder was dried and granulated, thereby obtaining a molding powder. Then, the molding powder was preliminarily molded, and molded by a hydrostatic press under a pressure of 3 ton/cm², thereby preparing a planar molding of 60×60×6 mm. Such moldings were fired in an atmosphere and at a temperature given in Table 1, thereby obtaining silicon nitride sintered body Nos. 1-12 according to the manufacturing process of the present invention. On the other hand, such moldings were fired in an atmosphere outside of the restricted range of the manufacturing process according to the present invention given in Table 1, thereby obtaining silicon nitride sintered body Nos. 13-24 as comparative examples. The firing atmosphere given in Table 1 were controlled by supplying N₂, CO₂ and CO raw gases to a firing furnace at rates given in Table 1.

With respect to the silicon nitride sintered body Nos. 1-24, Table 1 shows a bulk density, an oxygen content, a four point bending strength when a fired face or an inside worked face was employed as a tensile face, and increased amounts of oxidation per unit area in the fired face and the inside worked face when heated at 1,200° C. in air for 100 hours. The bulk density and the four point bending strength were measured according to an Archimedes method and "a fine ceramics bending strength testing method" of JIS R-1601, respectively, with respect to a fired face and an inside worked face worked as tensile face at a depth deeper than 1 mm from the surface. An increased amount of oxidation was determined from a weight increase and a surface area with respect to a sample having the whole surface fired and a sample having the whole surface which was an inside worked face worked at a depth not shallower than 1 mm from the surface after being heated in air.

                                      TABLE 1                                      __________________________________________________________________________                                                  Four point                                                                              Increased amount                      Firing conditions               bending strength                                                                        of oxidation                          Atmosphere                      (MPa)    (mg/cm.sup.2)                   Addition          Total                                                                               Temper-                                                                             Bulk Oxygen   Inside   Inside                      composition                                                                          CO.sub.2                                                                           CO Balance                                                                             pressure                                                                            ature                                                                               density                                                                             content                                                                             Fired                                                                              worked                                                                              Fired                                                                              worked               No.    (wt %)                                                                               (%) (%)                                                                               (%)  (atm)                                                                               (°C.)                                                                        (g/cm.sup.3)                                                                        (wt %)                                                                              face                                                                               face face                                                                               face                 __________________________________________________________________________     Present                                                                        invention                                                                       1     MgO(5)                                                                               0.001                                                                              0  N.sub.2                                                                              1   1750 3.18 4.2  680 710  0.8 0.8                   2     MgO(3)                                                                               0.1 0  N.sub.2                                                                             20   1900 3.20 3.3  820 880  0.6 0.6                   3     Al.sub.2 O.sub.3 (5)                                                                 10  1  N.sub.2                                                                             10   1850 3.16 3.8  650 700  0.1 0.1                   4     Al.sub.2 O.sub.3 (3)                                                                 20  2  N.sub.2                                                                             50   1900 3.15 3.0  720 800  0.1 0.1                   5     Y.sub.2 O.sub.3 (5)                                                                  0.1 0  N.sub. 2                                                                            10   1900 3.26 3.4  870 920  0.2 0.2                   6     Y.sub.2 O.sub.3 (3)                                                                  1   0.2                                                                               N.sub.2                                                                             100  2000 3.24 2.8  900 950  0.2 0.2                   7     Y.sub.2 O.sub.3 (5),                                                                 0.01                                                                               0  N.sub.2                                                                              1   1700 3.26 5.3  820 860  0.3 0.3                         MgO(5)                                                                   8     Y.sub.2 O.sub.3 (3),                                                                 1.5 0.1                                                                               N.sub.2                                                                             10   1850 3.28 4.4  970 1020 0.2 0.2                         MgO(3)                                                                   9     Y.sub.2 O.sub.3 (3),                                                                 50  0  N.sub.2 (50),                                                                       50   1900 3.29 4.2  900 990  0.2 0.2                         MgO(3)       Ar(50)                                                     10     Y.sub.2 O.sub.3 (5),                                                                 0.001                                                                              0  N.sub.2                                                                              1   1750 3.28 5.2  750 790  0.4 0.4                         Al.sub.2 O.sub.3 (5)                                                    11     Y.sub.2 O.sub.3 (3),                                                                 0.1 0.01                                                                              N.sub.2                                                                             300  1950 3.25 4.3  820 900  0.3 0.2                         Al.sub.2 O.sub.3 (3)                                                    12     Y.sub.2 O.sub.3 (3),                                                                 0.01                                                                               0  N.sub.2 (50),                                                                       10   1850 3.24 4.5  830 840  0.2 0.2                         Al.sub.2 O.sub.3 (3)                                                                        Ar(50)                                                     Comparative                                                                    example                                                                        13     MgO(5)                                                                               0   0  N.sub.2                                                                              1   1750 3.10 3.3  480 650  1.8 1.2                  14     MgO(3)                                                                               0   0  N.sub.2                                                                             20   1900 3.08 2.4  420 550  2.0 1.9                  15     Al.sub.2 O.sub.3 (5)                                                                 0   0  N.sub.2                                                                             10   1850 3.10 2.9  400 550  0.7 0.2                  16     Al.sub.2 O.sub.3 (3)                                                                 0   0  N.sub.2                                                                             50   1900 3.08 2.4  380 560  0.7 0.3                  17     Y.sub.2 O.sub.3 (5)                                                                  0   0  N.sub.2                                                                             10   1900 3.16 2.6  420 620  0.9 0.5                  18     Y.sub.2 O.sub.3 (3)                                                                  0   0  N.sub.2                                                                             100  2000 3.07 2.0  360 480  1.0 1.3                  19     Y.sub.2 O.sub.3 (5),                                                                 0   0  N.sub.2                                                                              1   1700 3.17 4.2  580 630  0.5 0.3                         MgO(5)                                                                  20     Y.sub.2 O.sub.3 (3),                                                                 0   0  N.sub.2                                                                             10   1850 3.19 3.4  560 680  0.9 0.2                         MgO(3)                                                                  21     Y.sub.2 O.sub.3 (3),                                                                 0   0  N.sub.2 (50),                                                                       50   1900 3.20 2.9  510 700  0.6 1.2                         MgO(3)       Ar(50)                                                     22     Y.sub.2 O.sub.3 (5),                                                                 0   0  N.sub.2                                                                              1   1750 3.22 4.0  470 700  0.8 0.6                         Al.sub.2 O.sub.3 (5)                                                    23     Y.sub.2 O.sub.3 (3),                                                                 0   0  N.sub.2                                                                             300  1950 3.19 2.8  480 670  1.1 0.8                         Al.sub.2 O.sub.3 (3)                                                    24     Y.sub.2 O.sub. 3 (3),                                                                0   0  N.sub.2 (50),                                                                       10   1850 3.19 2.9  510 650  0.9 0.4                         Al.sub.2 O.sub.3 (3)                                                                        Ar(50)                                                     __________________________________________________________________________

As evident from Table 1, as compared with the conventional processes, the sintered bodies having the same composition being denser and exhibiting the equal four point bending strength and the oxidation resistance both with respect to the fired surface and inside can be obtained by firing in the N₂ gas atmosphere or the mixed atmosphere of N₂ and an inert gas to which CO₂ or a mixed gas of CO₂ and CO is added.

The same moldings as in Sample No. 5 of the present invention in Example 1 were fired at 1900° C. in respective atmospheres containing CO and CO₂ shown in Table 2. As a result, sintered bodies each having approximately the same four point bending strength and increased amount of oxidation as in Sample No. 5 of the present invention with respect to the fired face and the inside worked face were obtained. At that time, a carbon pellet of 20 mm in diameter and 10 mm in height was placed and fired together with the silicon nitride sinterable body in the firing furnace. Reduced weights of carbon pellets due to the firing were shown in Table 2.

As understood from Table 2, the weight reducing of the carbon pellet in the firing is reduced by mixing CO to the firing atmosphere. That is, the damages of the carbonaceous heater or jig in the firing furnace can be reduced by mixing of CO.

                  TABLE 2                                                          ______________________________________                                                 Firing atmosphere   Reduced                                                                             Total  weight of                                        CO.sub.2                                                                               CO      Balance                                                                               pressure                                                                              carbon                                 No.       (%)     (%)     (%)    (atm)  pellet                                 ______________________________________                                         Present                                                                               25     0.1     0     N.sub.2                                                                               10     0.5                                  invention                                                                      Present                                                                               26     0.1     0.05  N.sub.2                                                                               10     0.4                                  invention                                                                      Present                                                                               27     0.1     0.1   N.sub.2                                                                               10     0.15                                 invention                                                                      Present                                                                               28     0.1     1     N.sub.2                                                                               10     0.05                                 invention                                                                      ______________________________________                                    

As obvious from the foregoing explanation, according to the silicon nitride-manufacturing process of the present invention, the silicon nitride sintered body which suffers almost no deterioration of the fired surface resulting from the evaporation and nitriding reaction of SiO₂, is dense, and exhibits the equal four bending strength and oxidation resistance with respect to the fired surface and the inside can be obtained by firing the molding in the N₂ atmosphere or the mixed atmosphere of N₂ and an inert gas to which CO₂ or a mixed gas of CO₂ and CO is added.

While there has been described preferred examples of the invention, obvious modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. 

What is claimed is:
 1. A process for manufacturing a silicon nitride sintered body comprising:forming a mold comprising a mixture of a silicon nitride starting material powder and a sintering aid; and firing said mold in at least one atmosphere selected from the group consisting of N₂ to which at least 0.001% of CO₂ is added, a mixture of N₂ and an inert gas to which at least 0.001% of CO₂ is added, and a mixture of N₂ and an inert gas to which a mixed gas of at least 0.001% of CO₂ and CO are added, wherein said at least one atmosphere contains a sufficient oxygen partial pressure to restrain the evaporation and nitriding of SiO₂ within said mold during firing.
 2. The process of claim 1, wherein the total pressure of the N₂ atmosphere and the mixed atmosphere of N₂ and the inert gas is not less than 1 atm.
 3. The process of claim 1, wherein the mixed gas of CO₂ and CO is mixed into at least one atmosphere selected from the group consisting of N₂ and a mixture of N₂ and an inert gas, such that a mixing rate of CO is greater than a mixing rate of CO₂.
 4. The process of claim 1, wherein a partial pressure of the mixed CO in the firing atmosphere is lower than an equilibrium CO partial pressure in the following formula:

    CO.sub.2 +C→CO. 